Analog-to-digital (A/D) converters are well-known electronic devices that are used in a wide variety of applications for converting an analog signal to a digital signal. A typical A/D converter includes some means for generating a group of reference voltages, a set of comparators for comparing the analog input with each of the reference voltages, and a circuit for encoding the outputs of the comparators as a digital value. FIG. 1A shows a simple A/D converter for generating a 2-bit digital output from an analog input signal V.sub.in. The analog input signal V.sub.in is supplied to a driver 2 for driving the input resistor-capacitor (RC) circuits of the converter. A network of resistors 3 connected in series to divide a reference voltage V.sub.R into three smaller reference voltages, respectively at "tap points" 4, 5, and 6. Each of the tap reference voltages is compared to the input voltage V.sub.in by a comparator 7. In this case, three comparators 7 are needed for the three reference voltages, to produce binary outputs V.sub.a, V.sub.b, and V.sub.c, respectively. For example, the comparators 7 produce a logical value "1" if the input signal V.sub.in is equal or greater than the tap reference voltage being compared to and a logical value "0" if the input signal is less than the compared reference voltage. An encoder 8 converts the outputs V.sub.a, V.sub.b, and V.sub.c into a 2-bit digital value according to some encoding scheme such as the one shown in FIG. 1B. Such an A/D converter is usually referred to as a single-ended or non-differential converter.
One of the problems of single-ended A/D converters is that they are highly susceptible to electrical noise such as that of the input signal, the power supply, and the switching of the reference voltages and comparators. To provide these converters with noise rejection capabilities, some prior art A/D converters were designed to receive a differential input such as the A/D converter shown in FIG. 2. The input stage of a conventional differential A/D converter of FIG. 2 typically includes a pair of drivers 20 for receiving the positive signal V.sub.in.sup.+ and negative signal V.sub.in.sup.- of the differential input. The incoming analog differential signals V.sub.in.sup.+ and V.sub.in.sup.- drive a pair of resistor ladders 21, respectively. Each of the resistor ladders 21 has a number of tap points 22 from which reference voltage levels are provided for comparing with the analog differential input signals. The voltage at each tap point 22 of the resistor ladders 21 follows the analog input signal that drives the respective resistor ladder, and is offset from the input signal by a fixed amount equal to the voltage drop across one or more resistors of the ladders.
The voltages from the taps 22 of the resistor ladders 21 are compared in parallel by a group of comparators 23 to generate outputs 24. Each comparator 23 compares the potential of a tap on a resistor ladder 21 with that of an opposite tap on the other resistor ladder. The outputs from the comparators 23 are then encoded as a digital value by encoder 25, as in the case of the A/D converter of FIG. 1A. These resistor ladders are referred to as dynamic ladders because the tap reference voltage levels vary depending on the differential input voltages V.sub.in.sup.+ and V.sub.in.sup.-, and not on some fixed voltage levels as in the ladders of FIG. 1A.
A major disadvantage of conventional differential A/D converters with dynamic resistor ladders, such as the one in FIG. 2, is that they typically have a relatively high power consumption. This is because the input currents from the drivers 20 must travel through the entire resistor ladders 21, which may include a large number of resistors as in the case of a digital output having many bits. Secondly, because the linearity of the A/D converter depends on how accurately the potential at each tap point follows the input signal, the resistor ladders 21 must have a bandwidth much wider than the analog input signals V.sub.in.sup.+ and V.sub.in.sup.-. Accordingly, the values of the resistors in the ladders 21 are typically kept very small, such as about 10.OMEGA.. The resistor ladders with such low resistance values, however, will dissipate a large amount of power. As a result, prior art differential input A/D converters that are designed to operate at high frequencies tend to have resistive ladders that consume a lot of electrical power, typically about 50% of the total power of the A/D converter.
U.S. Pat. No. 5,231,399 to Gorman et al., issued Jul. 27, 1993, discloses a differential quantizer reference resistor ladder for use in such an A/D converter. The converter receives positive and negative input signals and quantizes the input signals into quantization levels using two resistive networks. The quantization levels from the two networks are respectively compared, with their outputs being encoded into a digital value. Like other prior art differential A/D converters, this converter also employs dynamic resistor ladders driven by the input signals, and therefore consumes a high amount of power during operation.
In other prior art A/D converters, attempts have been made to reduce the power dissipation in the resistor ladders by using static ladders, rather than the dynamic ladders shown in FIG. 2. Static resistor ladders are those coupled to fixed voltage levels and not driven by the input signal, similar to the ladders in FIG. 1A. Another example of an A/D converter with static resistive ladders is shown in FIG. 3. In this case, a resistive ladder 31 divides a fixed reference voltage V.sub.T, with respect to a fixed reference voltage V.sub.B, into multiple tap reference voltages which appear at tap points 32 of the ladder 31. Since the voltages V.sub.T and V.sub.B are fixed, the reference voltages of the ladders are also fixed, or static. A group of comparators 33 compare an analog input V.sub.in with the tap reference voltages in parallel. The binary outputs of comparators 33 are then encoded into a digital value by an encoder 34. Note that the analog input signal no longer drives the resister ladders 31 and can be fed directly to a common input of the comparators 33. The resistors of the ladders 31 therefore can have a high resistance value in order to keep the power consumed by the A/D converter to a minimum. Although such a single-ended A/D converter with a static resistor ladder generally does not consume as much power as the conventional differential A/D converter described with reference to FIG. 2 above, it is still very susceptible to electrical noise as in the case of the A/D converter shown in FIG. 1A.
U.S. Pat. No. 4,612,531 to Dingwall et al., issued Sep. 16, 1986, discloses such a single-ended non-differential A/D converter. The disclosed converter employs a two-step conversion (referred to as a multi-step flash converter) to subdivide a reference voltage into multiple voltage steps using a single static resistor ladder. Although such a multi-step conversion generally reduces the total power consumed by the A/D converter, it increases the time delay in the analog-to-digital conversion and is still susceptible to the electrical noise problem.
U.S. Pat. No. 5,164,728 to Matsusawa et al., issued Nov. 17, 1992, discloses another single-ended non-differential A/D converter, but with complementary interpolating voltage dividers. The single input signal and reference voltages are differentially amplified by differential converting circuits, with interpolation resistors between the outputs of the converting circuits, for improved accuracy and speed. Since the resistive ladders are dynamic and the converter only accepts a single input signal, the disclosed A/D converter still suffers the problems with electrical noise and high power consumption described above.
Therefore, there remains a need for an efficient A/D converter capable of accepting a differential input for minimizing electrical noise interference, and using static resistive ladders for generating the reference voltages to achieve a low power consumption.